Strange error with mandatory property value

Page have the mandatory keyword-property. Keyword is a custom property which uses legacy editor to value selection. Keywords will be listed from another database.

If admin creates a new page there will be no problems at all.

If editor makes a new page EPi will first ask page name and type. No problems yet. Then EPi will ask mandatory keyword value. There is "click the button to edit"- button after Keyword-header. When user clicks the button this kind of error will appear instead of legacy editor and keyword lists:

--------------------------------------------------------------------------------Access was denied to page . Missing required permissions to edit this page Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. wmmameException Details: EPiServer.Core.AccessDeniedException: Access was denied to page . Missing required permissions to edit this page

Source Error: An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.